The purpose of this research was to determine whether the tachycardia from cocaine was vagally mediated. It has been assumed that cocaine is sympathomimetic. our previous results with many other drugs of abuse led to the hypothesis that cocaine, in addition to the other abused drugs that we have studied, produces vagally mediated tachycardia. The subjects were 14 males with extensive histories of cocaine abuse. on separate days, they were given i.v. administrations of placebo (saline) and cocaine (20 mg and 40 mg). We recorded heart rate continuously before and after injection. The results indicated that the tachycardia from i.v. cocaine is substantial. These increases in heart rate were mirrored precisely by decreases in vagal tone and the 0.10 Hz. rhythm. This pattern of response was consistent with vagal mediation of the effect of cocaine on heart rate. The significance of these results is that cocaine appears not to be sympathomimetic at these doses, as has been assumed previously. In contrast, the autonomic effects of cocaine appear to be mediated parasympathetically.